The melt extrusion of non-fluorinated melt processible polymers into shaped structures such as tubing, pipe, wire coating or film is accomplished by well-known procedures wherein a rotating screw pushes a viscous polymer melt through an extruder barrel into a die in which the polymer is shaped to the desired form and is then subsequently cooled and solidified into a product having the general shape of the die.
In order to achieve low production costs, it is desirable to extrude the polymer at rapid rates. Higher extrusion rates may be readily obtained by increasing the rate of revolution of the extruder screw. However, this technique is subject to limitations imposed by the viscoelastic properties of the polymer substrate. Thus, at very high extrusion rates an unacceptable amount of thermal decomposition of the polymer can result. Further, extrudates having a rough surface are often obtained which can lead to formation of an undesirable pattern on the surface of the extrudate.
In Blatz, U.S. Pat. No. 3,125,547, it is disclosed that the use of 0.01-2.0 wt. % of a fluorocarbon polymer that is in a fluid state at the process temperature, such as a fluoroelastomer, will reduce die pressure in extrusions of non-fluorinated polymers such as high and low density polyethylenes and other polyolefins. Further, use of this additive allows significant increase in extrusion rates without melt fracture.
More recently, improved fluoropolymer process aid compositions have been disclosed in, for example, U.S. Pat. Nos. 4,855,360; 6,642,310 B2 and U.S. 2006/0025523 A1. These compositions contain both a fluoropolymer and an interfacial agent such as a polyalkylene oxide or a polycaprolactone. The number average molecular weight (Mn) of these interfacial agents may be anything between about 400 to over 200,000. Low Mn (<5000) polyethylene glycol (PEG) works especially well in some process aids to reduce the time required to eliminate melt fracture of extrudable compositions. However, the low Mn PEG is moderately soluble in water which can cause massing problems in humid environments. Low Mn PEG is also susceptible to production of very low molecular weight fragments through thermally induced oxidative decomposition, making it potentially unstable during processing of the non-fluorinated melt processible polymer. These very low molecular weight PEG fragments can impart undesirable odor or taste characteristics in the extrudate, or cause unreliable printing or sealing of extruded films as the fragments bloom out of the polyethylene host polymer.
Thus there is a need for a process aid composition which reduces melt fracture in non-fluorinated melt processible polymers without massing or decomposing.